Apache Core Features

AcceptFilter Directive

This directive enables operating system specific optimizations for a listening socket by the Protocol type. The basic premise is for the kernel to not send a socket to the server process until either data is received or an entire HTTP Request is buffered. Only FreeBSD's Accept Filters and Linux's more primitive TCP_DEFER_ACCEPT are currently supported.

The default values on FreeBSD are:

The httpready accept filter buffers entire HTTP requests at the kernel level. Once an entire request is received, the kernel then sends it to the server. See the accf_http(9) man page for more details. Since HTTPS requests are encrypted only the accf_data(9) filter is used.

The default values on Linux are:

Linux's TCP_DEFER_ACCEPT does not support buffering http requests. Any value besides none will enable TCP_DEFER_ACCEPT on that listener. For more details see the Linux tcp(7) man page.

Using none for an argument will disable any accept filters for that protocol. This is useful for protocols that require a server send data first, such as nntp:

See also

• Protocol

AcceptPathInfo Directive

This directive controls whether requests that contain trailing pathname information that follows an actual filename (or non-existent file in an existing directory) will be accepted or rejected. The trailing pathname information can be made available to scripts in the PATH_INFO environment variable.

For example, assume the location /test/ points to a directory that contains only the single file here.html. Then requests for /test/here.html/more and /test/nothere.html/more both collect /more as PATH_INFO.

The three possible arguments for the AcceptPathInfo directive are:

Off

A request will only be accepted if it maps to a literal path that exists. Therefore a request with trailing pathname information after the true filename such as /test/here.html/more in the above example will return a 404 NOT FOUND error.

On

A request will be accepted if a leading path component maps to a file that exists. The above example /test/here.html/more will be accepted if /test/here.html maps to a valid file.

Default

The treatment of requests with trailing pathname information is determined by the handler responsible for the request. The core handler for normal files defaults to rejecting PATH_INFO requests. Handlers that serve scripts, such as cgi-script and isapi-handler, generally accept PATH_INFO by default.

The primary purpose of the AcceptPathInfo directive is to allow you to override the handler's choice of accepting or rejecting PATH_INFO. This override is required, for example, when you use a filter, such as INCLUDES, to generate content based on PATH_INFO. The core handler would usually reject the request, so you can use the following configuration to enable such a script:

AccessFileName Directive

While processing a request the server looks for the first existing configuration file from this list of names in every directory of the path to the document, if distributed configuration files are enabled for that directory. For example:

before returning the document /usr/local/web/index.html, the server will read /.acl, /usr/.acl, /usr/local/.acl and /usr/local/web/.acl for directives, unless they have been disabled with

See also

• AllowOverride
• Configuration Files
• .htaccess Files

AddDefaultCharset Directive

This directive specifies a default value for the media type charset parameter (the name of a character encoding) to be added to a response if and only if the response's content-type is either text/plain or text/html. This should override any charset specified in the body of the response via a META element, though the exact behavior is often dependent on the user's client configuration. A setting of AddDefaultCharset Off disables this functionality. AddDefaultCharset On enables a default charset of iso-8859-1. Any other value is assumed to be the charset to be used, which should be one of the IANA registered charset values for use in MIME media types. For example:

AddDefaultCharset should only be used when all of the text resources to which it applies are known to be in that character encoding and it is too inconvenient to label their charset individually. One such example is to add the charset parameter to resources containing generated content, such as legacy CGI scripts, that might be vulnerable to cross-site scripting attacks due to user-provided data being included in the output. Note, however, that a better solution is to just fix (or delete) those scripts, since setting a default charset does not protect users that have enabled the "auto-detect character encoding" feature on their browser.

See also

• AddCharset

AddOutputFilterByType Directive

This directive activates a particular output filter for a request depending on the response MIME-type. Because of certain problems discussed below, this directive is deprecated. The same functionality is available using mod_filter.

The following example uses the DEFLATE filter, which is provided by mod_deflate. It will compress all output (either static or dynamic) which is labeled as text/html or text/plain before it is sent to the client.

If you want the content to be processed by more than one filter, their names have to be separated by semicolons. It's also possible to use one AddOutputFilterByType directive for each of these filters.

The configuration below causes all script output labeled as text/html to be processed at first by the INCLUDES filter and then by the DEFLATE filter.

See also

• AddOutputFilter
• SetOutputFilter
• filters

AllowEncodedSlashes Directive

The AllowEncodedSlashes directive allows URLs which contain encoded path separators (%2F for / and additionally %5C for \ on according systems) to be used in the path info.

With the default value, Off, such URLs are refused with a 404 (Not found) error.

With the value On, such URLs are accepted, and encoded slashes are decoded like all other encoded characters.

With the value NoDecode, such URLs are accepted, but encoded slashes are not decoded but left in their encoded state.

Turning AllowEncodedSlashes On is mostly useful when used in conjunction with PATH_INFO.

See also

• AcceptPathInfo

AllowOverride Directive

When the server finds an .htaccess file (as specified by AccessFileName) it needs to know which directives declared in that file can override earlier configuration directives.

When this directive is set to None, then .htaccess files are completely ignored. In this case, the server will not even attempt to read .htaccess files in the filesystem.

When this directive is set to All, then any directive which has the .htaccess Context is allowed in .htaccess files.

The directive-type can be one of the following groupings of directives.

AuthConfig

Allow use of the authorization directives (AuthDBMGroupFile, AuthDBMUserFile, AuthGroupFile, AuthName, AuthType, AuthUserFile, Require, etc.).

FileInfo

Allow use of the directives controlling document types (DefaultType, ErrorDocument, ForceType, LanguagePriority, SetHandler, SetInputFilter, SetOutputFilter, and mod_mime Add* and Remove* directives, etc.), document meta data (Header, RequestHeader, SetEnvIf, SetEnvIfNoCase, BrowserMatch, CookieExpires, CookieDomain, CookieStyle, CookieTracking, CookieName), mod_rewrite directives (RewriteEngine, RewriteOptions, RewriteBase, RewriteCond, RewriteRule), mod_alias directives (Redirect, RedirectTemp, RedirectPermanent, RedirectMatch), and Action from mod_actions.

Indexes

Allow use of the directives controlling directory indexing (AddDescription, AddIcon, AddIconByEncoding, AddIconByType, DefaultIcon, DirectoryIndex, FancyIndexing , HeaderName, IndexIgnore, IndexOptions, ReadmeName, etc.).

Limit

Allow use of the directives controlling host access (Allow, Deny and Order).

Options[=Option,...]

Allow use of the directives controlling specific directory features (Options and XBitHack). An equal sign may be given followed by a comma (but no spaces) separated lists of options that may be set using the Options command.

Implicit disabling of Options

Even though the list of options that may be used in .htaccess files can be limited with this directive, as long as any Options directive is allowed any other inherited option can be disabled by using the non-relative syntax. In other words, this mechanism cannot force a specific option to remain set while allowing any others to be set.

Example:

In the example above all directives that are neither in the group AuthConfig nor Indexes cause an internal server error.

See also

• AccessFileName
• Configuration Files
• .htaccess Files

AuthName Directive

This directive sets the name of the authorization realm for a directory. This realm is given to the client so that the user knows which username and password to send. AuthName takes a single argument; if the realm name contains spaces, it must be enclosed in quotation marks. It must be accompanied by AuthType and Require directives, and directives such as AuthUserFile and AuthGroupFile to work.

For example:

The string provided for the AuthName is what will appear in the password dialog provided by most browsers.

See also

• Authentication, Authorization, and Access Control

AuthType Directive

This directive selects the type of user authentication for a directory. The authentication types available are Basic (implemented by mod_auth_basic) and Digest (implemented by mod_auth_digest).

To implement authentication, you must also use the AuthName and Require directives. In addition, the server must have an authentication-provider module such as mod_authn_file and an authorization module such as mod_authz_user.

See also

• Authentication and Authorization
• Access Control

CGIMapExtension Directive

This directive is used to control how Apache finds the interpreter used to run CGI scripts. For example, setting CGIMapExtension sys:\foo.nlm .foo will cause all CGI script files with a .foo extension to be passed to the FOO interpreter.

ContentDigest Directive

This directive enables the generation of Content-MD5 headers as defined in RFC1864 respectively RFC2616.

MD5 is an algorithm for computing a "message digest" (sometimes called "fingerprint") of arbitrary-length data, with a high degree of confidence that any alterations in the data will be reflected in alterations in the message digest.

The Content-MD5 header provides an end-to-end message integrity check (MIC) of the entity-body. A proxy or client may check this header for detecting accidental modification of the entity-body in transit. Example header:

Note that this can cause performance problems on your server since the message digest is computed on every request (the values are not cached).

Content-MD5 is only sent for documents served by the core, and not by any module. For example, SSI documents, output from CGI scripts, and byte range responses do not have this header.

DefaultType Directive

There will be times when the server is asked to provide a document whose type cannot be determined by its MIME types mappings.

The server SHOULD inform the client of the content-type of the document. If the server is unable to determine this by normal means, it will set it to the configured DefaultType. For example:

would be appropriate for a directory which contained many GIF images with filenames missing the .gif extension.

In cases where it can neither be determined by the server nor the administrator (e.g. a proxy), it is preferable to omit the MIME type altogether rather than provide information that may be false. This can be accomplished using

DefaultType None is only available in httpd-2.2.7 and later.

Note that unlike ForceType, this directive only provides the default mime-type. All other mime-type definitions, including filename extensions, that might identify the media type will override this default.

<Directory> Directive

<Directory> and </Directory> are used to enclose a group of directives that will apply only to the named directory, sub-directories of that directory, and the files within the respective directories. Any directive that is allowed in a directory context may be used. Directory-path is either the full path to a directory, or a wild-card string using Unix shell-style matching. In a wild-card string, ? matches any single character, and * matches any sequences of characters. You may also use [] character ranges. None of the wildcards match a `/' character, so <Directory /*/public_html> will not match /home/user/public_html, but <Directory /home/*/public_html> will match. Example:

Regular expressions can also be used, with the addition of the ~ character. For example:

would match directories in /www/ that consisted of three numbers.

If multiple (non-regular expression) <Directory> sections match the directory (or one of its parents) containing a document, then the directives are applied in the order of shortest match first, interspersed with the directives from the .htaccess files. For example, with

for access to the document /home/web/dir/doc.html the steps are:

• Apply directive AllowOverride None (disabling .htaccess files).
• Apply directive AllowOverride FileInfo (for directory /home).
• Apply any FileInfo directives in /home/.htaccess, /home/web/.htaccess and /home/web/dir/.htaccess in that order.

Regular expressions are not considered until after all of the normal sections have been applied. Then all of the regular expressions are tested in the order they appeared in the configuration file. For example, with

the regular expression section won't be considered until after all normal <Directory>s and .htaccess files have been applied. Then the regular expression will match on /home/abc/public_html/abc and the corresponding <Directory> will be applied.

Note that the default Apache access for <Directory /> is Allow from All. This means that Apache will serve any file mapped from an URL. It is recommended that you change this with a block such as

and then override this for directories you want accessible. See the Security Tips page for more details.

The directory sections occur in the httpd.conf file. <Directory> directives cannot nest, and cannot appear in a <Limit> or <LimitExcept> section.

See also

• How <Directory>, <Location> and <Files> sections work for an explanation of how these different sections are combined when a request is received

<DirectoryMatch> Directive

<DirectoryMatch> and </DirectoryMatch> are used to enclose a group of directives which will apply only to the named directory and sub-directories of that directory (and the files within), the same as <Directory>. However, it takes as an argument a regular expression. For example:

would match directories in /www/ that consisted of three numbers.

See also

• <Directory> for a description of how regular expressions are mixed in with normal <Directory>s
• How <Directory>, <Location> and <Files> sections work for an explanation of how these different sections are combined when a request is received

DocumentRoot Directive

This directive sets the directory from which httpd will serve files. Unless matched by a directive like Alias, the server appends the path from the requested URL to the document root to make the path to the document. Example:

then an access to http://www.my.host.com/index.html refers to /usr/web/index.html. If the directory-path is not absolute then it is assumed to be relative to the ServerRoot.

The DocumentRoot should be specified without a trailing slash.

See also

• Mapping URLs to Filesystem Locations

EnableMMAP Directive

This directive controls whether the httpd may use memory-mapping if it needs to read the contents of a file during delivery. By default, when the handling of a request requires access to the data within a file -- for example, when delivering a server-parsed file using mod_include -- Apache memory-maps the file if the OS supports it.

This memory-mapping sometimes yields a performance improvement. But in some environments, it is better to disable the memory-mapping to prevent operational problems:

• On some multiprocessor systems, memory-mapping can reduce the performance of the httpd.
• Deleting or truncating a file while httpd has it memory-mapped can cause httpd to crash with a segmentation fault.

For server configurations that are vulnerable to these problems, you should disable memory-mapping of delivered files by specifying:

For NFS mounted files, this feature may be disabled explicitly for the offending files by specifying:

EnableSendfile Directive

This directive controls whether httpd may use the sendfile support from the kernel to transmit file contents to the client. By default, when the handling of a request requires no access to the data within a file -- for example, when delivering a static file -- Apache uses sendfile to deliver the file contents without ever reading the file if the OS supports it.

This sendfile mechanism avoids separate read and send operations, and buffer allocations. But on some platforms or within some filesystems, it is better to disable this feature to avoid operational problems:

• Some platforms may have broken sendfile support that the build system did not detect, especially if the binaries were built on another box and moved to such a machine with broken sendfile support.
• On Linux the use of sendfile triggers TCP-checksum offloading bugs on certain networking cards when using IPv6.
• On Linux on Itanium, sendfile may be unable to handle files over 2GB in size.
• With a network-mounted DocumentRoot (e.g., NFS or SMB), the kernel may be unable to serve the network file through its own cache.

For server configurations that are vulnerable to these problems, you should disable this feature by specifying:

For NFS or SMB mounted files, this feature may be disabled explicitly for the offending files by specifying:

Please note that the per-directory and .htaccess configuration of EnableSendfile is not supported by mod_disk_cache. Only global definition of EnableSendfile is taken into account by the module.

ErrorDocument Directive

In the event of a problem or error, Apache can be configured to do one of four things,

1. output a simple hardcoded error message
2. output a customized message
3. internally redirect to a local URL-path to handle the problem/error
4. redirect to an external URL to handle the problem/error

The first option is the default, while options 2-4 are configured using the ErrorDocument directive, which is followed by the HTTP response code and a URL or a message. Apache will sometimes offer additional information regarding the problem/error.

URLs can begin with a slash (/) for local web-paths (relative to the DocumentRoot), or be a full URL which the client can resolve. Alternatively, a message can be provided to be displayed by the browser. Examples:

Additionally, the special value default can be used to specify Apache's simple hardcoded message. While not required under normal circumstances, default will restore Apache's simple hardcoded message for configurations that would otherwise inherit an existing ErrorDocument.

Note that when you specify an ErrorDocument that points to a remote URL (ie. anything with a method such as http in front of it), Apache will send a redirect to the client to tell it where to find the document, even if the document ends up being on the same server. This has several implications, the most important being that the client will not receive the original error status code, but instead will receive a redirect status code. This in turn can confuse web robots and other clients which try to determine if a URL is valid using the status code. In addition, if you use a remote URL in an ErrorDocument 401, the client will not know to prompt the user for a password since it will not receive the 401 status code. Therefore, if you use an ErrorDocument 401 directive then it must refer to a local document.

Microsoft Internet Explorer (MSIE) will by default ignore server-generated error messages when they are "too small" and substitute its own "friendly" error messages. The size threshold varies depending on the type of error, but in general, if you make your error document greater than 512 bytes, then MSIE will show the server-generated error rather than masking it. More information is available in Microsoft Knowledge Base article Q294807.

Although most error messages can be overridden, there are certain circumstances where the internal messages are used regardless of the setting of ErrorDocument. In particular, if a malformed request is detected, normal request processing will be immediately halted and the internal error message returned. This is necessary to guard against security problems caused by bad requests.

If you are using mod_proxy, you may wish to enable ProxyErrorOverride so that you can provide custom error messages on behalf of your Origin servers. If you don't enable ProxyErrorOverride, Apache will not generate custom error documents for proxied content.

Prior to version 2.0, messages were indicated by prefixing them with a single unmatched double quote character.

See also

• documentation of customizable responses

ErrorLog Directive

The ErrorLog directive sets the name of the file to which the server will log any errors it encounters. If the file-path is not absolute then it is assumed to be relative to the ServerRoot.

If the file-path begins with a pipe character "|" then it is assumed to be a command to spawn to handle the error log.

See the notes on piped logs for more information.

Using syslog instead of a filename enables logging via syslogd(8) if the system supports it. The default is to use syslog facility local7, but you can override this by using the syslog:facility syntax where facility can be one of the names usually documented in syslog(1).

SECURITY: See the security tips document for details on why your security could be compromised if the directory where log files are stored is writable by anyone other than the user that starts the server.

See also

• LogLevel
• Apache Log Files

FileETag Directive

The FileETag directive configures the file attributes that are used to create the ETag (entity tag) response header field when the document is based on a static file. (The ETag value is used in cache management to save network bandwidth.) In Apache 1.3.22 and earlier, the ETag value was always formed from the file's inode, size, and last-modified time (mtime). The FileETag directive allows you to choose which of these -- if any -- should be used. The recognized keywords are:

INode

The file's i-node number will be included in the calculation

MTime

The date and time the file was last modified will be included

Size

The number of bytes in the file will be included

All

All available fields will be used. This is equivalent to:

FileETag INode MTime Size

None

If a document is file-based, no ETag field will be included in the response

The INode, MTime, and Size keywords may be prefixed with either + or -, which allow changes to be made to the default setting inherited from a broader scope. Any keyword appearing without such a prefix immediately and completely cancels the inherited setting.

If a directory's configuration includes FileETag INode MTime Size, and a subdirectory's includes FileETag -INode, the setting for that subdirectory (which will be inherited by any sub-subdirectories that don't override it) will be equivalent to FileETag MTime Size.

<Files> Directive

The <Files> directive limits the scope of the enclosed directives by filename. It is comparable to the <Directory> and <Location> directives. It should be matched with a </Files> directive. The directives given within this section will be applied to any object with a basename (last component of filename) matching the specified filename. <Files> sections are processed in the order they appear in the configuration file, after the <Directory> sections and .htaccess files are read, but before <Location> sections. Note that <Files> can be nested inside <Directory> sections to restrict the portion of the filesystem they apply to.

The filename argument should include a filename, or a wild-card string, where ? matches any single character, and * matches any sequences of characters:

<Files "cat.html">
    # Insert stuff that applies to cat.html here
</Files>

<Files "?at.*">
    # This would apply to cat.html, bat.html, hat.php and so on.
</Files>

Regular expressions can also be used, with the addition of the ~ character. For example:

would match most common Internet graphics formats. <FilesMatch> is preferred, however.

Note that unlike <Directory> and <Location> sections, <Files> sections can be used inside .htaccess files. This allows users to control access to their own files, at a file-by-file level.

See also

• How <Directory>, <Location> and <Files> sections work for an explanation of how these different sections are combined when a request is received

<FilesMatch> Directive

The <FilesMatch> directive limits the scope of the enclosed directives by filename, just as the <Files> directive does. However, it accepts a regular expression. For example:

would match most common Internet graphics formats.

See also

• How <Directory>, <Location> and <Files> sections work for an explanation of how these different sections are combined when a request is received

ForceType Directive

When placed into an .htaccess file or a <Directory>, or <Location> or <Files> section, this directive forces all matching files to be served with the content type identification given by MIME-type. For example, if you had a directory full of GIF files, but did not want to label them all with .gif, you might want to use:

Note that unlike DefaultType, this directive overrides all mime-type associations, including filename extensions, that might identify the media type.

You can override any ForceType setting by using the value of None:

GprofDir Directive

When the server has been compiled with gprof profiling support, GprofDir causes gmon.out files to be written to the specified directory when the process exits. If the argument ends with a percent symbol ('%'), subdirectories are created for each process id.

This directive currently only works with the prefork MPM.

HostnameLookups Directive

This directive enables DNS lookups so that host names can be logged (and passed to CGIs/SSIs in REMOTE_HOST). The value Double refers to doing double-reverse DNS lookup. That is, after a reverse lookup is performed, a forward lookup is then performed on that result. At least one of the IP addresses in the forward lookup must match the original address. (In "tcpwrappers" terminology this is called PARANOID.)

Regardless of the setting, when mod_authz_host is used for controlling access by hostname, a double reverse lookup will be performed. This is necessary for security. Note that the result of this double-reverse isn't generally available unless you set HostnameLookups Double. For example, if only HostnameLookups On and a request is made to an object that is protected by hostname restrictions, regardless of whether the double-reverse fails or not, CGIs will still be passed the single-reverse result in REMOTE_HOST.

The default is Off in order to save the network traffic for those sites that don't truly need the reverse lookups done. It is also better for the end users because they don't have to suffer the extra latency that a lookup entails. Heavily loaded sites should leave this directive Off, since DNS lookups can take considerable amounts of time. The utility logresolve, compiled by default to the bin subdirectory of your installation directory, can be used to look up host names from logged IP addresses offline.

<IfDefine> Directive